De-catting

[bumcrack]

Thinking of decatting my 996, has any one gone down a similar route, or maybe headers?

www.design911.co.uk/index.htm?Region=UK

Click on Exhaust for 996

I've decatted before and been happy with the results in the past. With a map, headers, and decat you could looking at a gain in the region of 40BHP


Anyone else?

[MOD500]

If you go down this route make sure you keep all the original bits and pieces, as come selling time some buyers may be put off by a loud sports exhaust.

Another firm that does this kind of thing is Fabspeed:

www.fabspeed.com

Mind you I lost interest in looking on the site for your exhausts when I saw those ladies with the 'Hotwheels' jump suits on

Hope you get fixed up.


Martyn.

[nel]

If you de-cat a car, do you have to refit the old system for the purposes of MOT testing. Obviously I don't mean for the sound, rather for the emissions levels.

[bumcrack]

MOD500,

The products on the site are Fabspeed products

I'm not really worried about having to reinstall the cats for the MOT, I just wondered if any one else has removed the cats on the 996 and gained any BHP, the web site quotes 9 BHP, but I'm sceptical. You'll have less back pressure and may lose torque

>> Edited by bumcrack on Sunday 17th October 22:15

[MOD500]

Not sure about the power gains you may get from de-catting, prob best to talk to someone like ninemeister on here who really knows the craic.

If chipping is on the cards best to talk to DMS or AMD, etc.

www.dmsautomotive.com

www.auto-amd.com

They offer sports exhausts also, which I think are chosen to compliment the re-mapped chip well.

Hope that helps


Martyn.

Edited for url madness.

>> Edited by MOD500 on Tuesday 19th October 08:57

>> Edited by MOD500 on Tuesday 19th October 08:58

[Marquis_Rex]

bumcrack said:

Thinking of decatting my 996, has any one gone down a similar route, or maybe headers? www.design911.co.uk/index.htm?Region=UK Click on Exhaust for 996 I've decatted before and been happy with the results in the past. With a map, headers, and decat you could looking at a gain in the region of 40BHP Anyone else?

When remapping, make sure all the OBD2 trips are deactivated, including the UHEGO (upstream heated exhaust gas oxygen sensor) and HEGO sensors, which montior the condition of the first brick and put up an engine "mil" light if the catalyst efficiency drops. 40 Bhp increase for a de-catt and re map is wildly optimistic for the naturally aspirated 996 motor. The problem is that there are too many wild "pub talk" claims out there, more often then not by garages involved with tuning (who may other wise be good). It is doubtful any garage has gone through engines, put a static pressure tapping on the exhaust (pre and post cat) and measured the output of the engine before and after on a DIN70020 calibrated dyno. Incremental builds are often best for this sort of evaluation.

I haven't had direct dyno experience of this flat six engine but to give some examples to illustrate:
A racing V8 engine I helped develop dropped power by less than 10 Bhp when the back pressure dropped from 430 mbar to 230 mbar. So the back pressure almost HALVED! Valve overlap is what gives an engine sensitivity to back pressure, and this racing engine had ALOT more valve overlap area then a tame road going 996 engine.
To illustrate the dependence on valve overlap on exhaust backpressure sensitivity, anoher example is:
A supercharged engine gained only 3 Bhp when the back pressure reduced by 200 mbar! This is because the overlap was alot less on this engine.
A 996 is a 4 valver with quite tame low-valve overlap timings. Losing the catalysts altogether may lose you 200 mbar absolute tops. The 996 catalysts are already low back pressure metal matrix affairs so I imagine you'd reduce back pressure less in fact.

Moral of the story is that you can't generalise when it comes to engines and there is alot of crap perpetuated by mechanics and technicians, and optimistic wishfuls. It doesn't matter how succesful ones reputation is in racing or how good their servicing and other technical knowledge is, there is no substitute in getting data from a test bed engine or talking to a development engineer.

[Marquis_Rex]

bumcrack said:

MOD500, The products on the site are Fabspeed products I'm not really worried about having to reinstall the cats for the MOT, I just wondered if any one else has removed the cats on the 996 and gained any BHP, the web site quotes 9 BHP, but I'm sceptical. You'll have less back pressure and may lose torque >> Edited by bumcrack on Sunday 17th October 22:15

Less backpressure will NOT lose you torque.
I don't know where this myth comes from. I imagine it's confusion from TUNING effects and back pressure. Going from a larger system diameter to a smaller diameter WILL gain you a little torque at VERY low speeds (depending on where the system was reduced) and you would lose power due to the higher restriction- but this isn't the same thing as gaining torque DUE to the increased restriction.
High backpressure will cause pumping losses to increase and hence torque output to diminish.